I'm not sure he deserves to be on the Wall but I would not complain if he was.
It wasn't like he played just that one game. He led an incredible comeback against the Rams on MNF in 1989 not once but twice in the last two minutes -- after he led what appeared to be the game-winning TD drive, Everett hit Flipper Anderson with something like an 80-yard TD pass to put the Rams ahead, and Frank calmly led the Bills back down the field for the winning TD with something like 16 seconds left.
He also won playoff games against the Dolphins in 1991 and the Steelers (in Pittsburgh) in 1993 with Kelly on the shelf. A very good case can be made for saying that the Bills wouldn't even have gone to TWO straight Super Bowls had Reich not been around. If we'd had a pedestrian backup in 1990 and 1992, we might have only gone to SBs XXVI and XXVIII.
I think there is also something to be said for what a pure class act he was. As well as he played in his limited engagements and as popular as he was with the fans (remember that infamous D&C poll?), he never said anything other than "I'm just keeping the seat warm for Jim," and he never once intimated a desire to go elsewhere a la the Scott Mitchells of the world.